Implement shrinking reductions in CPS IR

sdk/lib/_internal/compiler/implementation/cps_ir/shrinking_reductions.dart

+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+part of dart2js.cps_optimizers;
+
+/**
+ * [[ShrinkingReducer]] applies shrinking reductions to CPS terms as described
+ * in 'Compiling with Continuations, Continued' by Andrew Kennedy.
+ */
+class ShrinkingReducer implements Pass {
+  _RedexVisitor _redexVisitor;
+  Set<_ReductionTask> _worklist;
+
+  static final _DeletedNode _DELETED = new _DeletedNode();
+
+  /// Applies shrinking reductions to root, mutating root in the process.
+  void rewrite(FunctionDefinition root) {
+    _worklist = new Set<_ReductionTask>();
+    _redexVisitor = new _RedexVisitor(_worklist);
+
+    // Set all parent pointers.
+    new _ParentVisitor().visit(root);
+
+    // Sweep over the term, collecting redexes into the worklist.
+    _redexVisitor.visitFunctionDefinition(root);
+
+    // Process the worklist.
+    while (_worklist.isNotEmpty) {
+      _ReductionTask task = _worklist.first;
+      _worklist.remove(task);
+      _processTask(task);
+    }
+  }
+
+  void _processTask(_ReductionTask task) {
+    // Lazily skip tasks for deleted nodes.
+    if (task.node.parent == _DELETED) {
+      return;
+    }
+
+    switch (task.kind) {
+      case _ReductionKind.DEAD_VAL:
+        _reduceDeadVal(task);
+        break;
+      case _ReductionKind.DEAD_CONT:
+        _reduceDeadCont(task);
+        break;
+      case _ReductionKind.BETA_CONT_LIN:
+        _reduceBetaContLin(task);
+        break;
+      case _ReductionKind.ETA_CONT:
+        _reduceEtaCont(task);
+        break;
+      default:
+        assert(false);
+    }
+  }
+
+  /// Applies the dead-val reduction:
+  ///   letprim x = V in K -> K (x not free in K).
+  void _reduceDeadVal(_ReductionTask task) {
+    if (!_redexVisitor.isDeadVal(task.node)) {

[sigurdm, 2014/08/01 09:09:41]

Add comment why this can happen

[jgruber1, 2014/08/07 09:07:36]

Done, and replaced by assert in dead-* reductions.

+      return;
+    }
+
+    LetPrim letPrim = task.node;
+    NodeWithBody parent = letPrim.parent;
+    assert(letPrim != null && parent != null);
+
+    assert(!letPrim.primitive.hasAtLeastOneUse);
+
+    // Remove dead primitive.
+    letPrim.body.parent = parent;
+    parent.body = letPrim.body;
+
+    // Perform bookkeeping on removed body and scan for new redexes.
+    new _RemovalRedexVisitor(_worklist).visit(letPrim.primitive);
+  }
+
+  /// Applies the dead-cont reduction:
+  ///   letcont k x = L in K -> K (k not free in K).
+  void _reduceDeadCont(_ReductionTask task) {
+    if (!_redexVisitor.isDeadCont(task.node)) {
+      return;
+    }
+
+    LetCont letCont = task.node;
+    NodeWithBody parent = letCont.parent;
+    assert(letCont != null && parent != null);
+
+    assert(!letCont.continuation.hasAtLeastOneUse);
+
+    // Remove dead continuation.
+    letCont.body.parent = parent;
+    parent.body = letCont.body;
+
+    // Perform bookkeeping on removed body and scan for new redexes.
+    new _RemovalRedexVisitor(_worklist).visit(letCont.continuation);
+  }
+
+  /// Applies the beta-cont-lin reduction:
+  ///   letcont k x = K in C[k y] -> C[K[y/x]] (k not free in C).
+  void _reduceBetaContLin(_ReductionTask task) {
+    if (!_redexVisitor.isBetaContLin(task.node)) {
+      return;
+    }
+
+    LetCont letCont = task.node;
+    Continuation cont = letCont.continuation;
+    NodeWithBody parent = letCont.parent;
+    assert(letCont != null && parent != null);
+
+    assert(cont.hasExactlyOneUse);
+
+    // Remove the continuation.
+    letCont.body.parent = parent;

[sigurdm, 2014/08/01 09:09:41]

You might be able to factor these two lines out.

[jgruber1, 2014/08/07 09:07:36]

Done.

+    parent.body = letCont.body;
+
+    // Replace its invocation with the continuation body.
+    Reference ref = cont.firstRef..unlink();
+    InvokeContinuation invoke = ref.parent;
+    NodeWithBody invokeParent = invoke.parent;
+    assert(invoke != null && invokeParent != null);
+
+    cont.body.parent = invokeParent;
+    invokeParent.body = cont.body;
+
+    // Substitute the invocation argument for the continuation parameter.
+    for (int i = 0; i < invoke.arguments.length; i++) {
+      Reference argRef = invoke.arguments[i]..unlink();
+      argRef.definition.substituteFor(cont.parameters[i]);
+      // Scan for new redexes in substituted references.
+      _redexVisitor.processReference(argRef);
+    }
+
+    // Do not scan for new redexes in the continuation body to avoid quadratic
+    // blowup.
+  }
+
+  /// Applies the eta-cont reduction:
+  ///   letcont k x = j x in K -> K[j/k].
+  void _reduceEtaCont(_ReductionTask task) {
+    if (!_redexVisitor.isEtaCont(task.node)) {
+      return;
+    }
+
+    LetCont letCont = task.node;
+    Continuation cont = letCont.continuation;
+    NodeWithBody parent = letCont.parent;
+    assert(letCont != null && parent != null);
+
+    assert(cont.hasAtLeastOneUse);
+
+    // Remove the continuation.
+    letCont.body.parent = parent;
+    parent.body = letCont.body;
+
+    InvokeContinuation invoke = cont.body;
+    Continuation wrappedCont = invoke.continuation.definition;
+
+    // Replace all occurrences with the wrapped continuation.
+    wrappedCont.substituteFor(cont);
+    _redexVisitor.processReference(invoke.continuation);
+
+    // Do not scan for new redexes in the letcont body to avoid quadratic
+    // blowup.
+  }
+}
+
+/// Traverses a term and adds any found redexes to the worklist.
+class _RedexVisitor extends RecursiveVisitor {
+  final Set<_ReductionTask> worklist;
+
+  _RedexVisitor(this.worklist);
+
+  processLetPrim(LetPrim node) {
+    if (isDeadVal(node)) {
+      worklist.add(new _ReductionTask(_ReductionKind.DEAD_VAL, node));
+    }
+  }
+
+  processLetCont(LetCont node) {
+    if (isDeadCont(node)) {
+      worklist.add(new _ReductionTask(_ReductionKind.DEAD_CONT, node));
+    }
+    if (isBetaContLin(node)){
+      worklist.add(new _ReductionTask(_ReductionKind.BETA_CONT_LIN, node));
+    }
+    if (isEtaCont(node)) {
+      worklist.add(new _ReductionTask(_ReductionKind.ETA_CONT, node));
+    }
+  }
+
+  processReference(Reference reference) {
+    if (reference.definition is Primitive) {
+      Primitive primitive = reference.definition;
+      Node parent = primitive.parent;
+      if (parent is LetPrim && isDeadVal(parent)) {
+        worklist.add(new _ReductionTask(_ReductionKind.DEAD_VAL, parent));
+      }
+    } else if (reference.definition is Continuation) {
+      Continuation continuation = reference.definition;
+      Node parent = continuation.parent;
+      if (parent is LetCont && isDeadCont(parent)) {
+        worklist.add(new _ReductionTask(_ReductionKind.DEAD_CONT, parent));
+      }
+    }
+  }
+
+  bool isDeadVal(LetPrim node) => !node.primitive.hasAtLeastOneUse;
+
+  bool isDeadCont(LetCont node) => !node.continuation.hasAtLeastOneUse;
+
+  bool isBetaContLin(LetCont node) {
+    Continuation cont = node.continuation;
+    if (!cont.hasExactlyOneUse) {
+      return false;
+    }
+
+    if (!(cont.firstRef.parent is InvokeContinuation)) {
+      return false;
+    }
+
+    InvokeContinuation invoke = cont.firstRef.parent;
+    return (cont == invoke.continuation.definition);
+  }
+
+  bool isEtaCont(LetCont node) {
+    Continuation cont = node.continuation;
+    if (!(cont.body is InvokeContinuation)) {
+      return false;
+    }
+
+    // Special case for continuations passed into one of { InvokeConstructor,
+    // InvokeMethod, InvokeStatic, ConcatenateStrings }, since
+    // these require a continuation that is used exactly once.
+    if (cont.hasExactlyOneUse) {
+      if (cont.firstRef.parent is InvokeConstructor) {
+        InvokeConstructor parent = cont.firstRef.parent;
+        if (parent.continuation == cont.firstRef) {
+          return false;
+        }
+      } else if (cont.firstRef.parent is InvokeMethod) {
+        InvokeMethod parent = cont.firstRef.parent;
+        if (parent.continuation == cont.firstRef) {
+          return false;
+        }
+      } else if (cont.firstRef.parent is InvokeStatic) {
+        InvokeStatic parent = cont.firstRef.parent;
+        if (parent.continuation == cont.firstRef) {
+          return false;
+        }
+      } else if (cont.firstRef.parent is ConcatenateStrings) {
+        ConcatenateStrings parent = cont.firstRef.parent;
+        if (parent.continuation == cont.firstRef) {
+          return false;
+        }
+      }
+    }
+
+    InvokeContinuation invoke = cont.body;
+    if (invoke.isRecursive) {
+      return false;
+    }
+
+    if (cont.parameters.length != invoke.arguments.length) {
+      return false;
+    }
+
+    // TODO(jgruber): Linear in the parameter count. Can be improved to near
+    // constant time by using union-find data structure.
+    for (int i = 0; i < cont.parameters.length; i++) {
+      if (invoke.arguments[i].definition != cont.parameters[i]) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+}
+
+/// Traverses a deleted CPS term, marking existing tasks associated with a node
+/// within the term as deleted (which causes them to be skipped lazily when
+/// popped from the worklist), and adding newly created redexes to the worklist.
+class _RemovalRedexVisitor extends _RedexVisitor {
+  _RemovalRedexVisitor(Set<_ReductionTask> worklist) : super(worklist);
+
+  processLetPrim(LetPrim node) {
+    node.parent = ShrinkingReducer._DELETED;
+  }
+
+  processLetCont(LetCont node) {
+    node.parent = ShrinkingReducer._DELETED;
+  }
+
+  processReference(Reference reference) {
+    reference.unlink();
+    super.processReference(reference); // Scan for new dead-* redexes.
+  }
+}
+
+/// Traverses the CPS term and sets node.parent for each visited node.
+class _ParentVisitor extends RecursiveVisitor {
+
+  void setParent(Node parent, Node child) {
+    assert(child.parent == null);
+    child.parent = parent;
+  }
+
+  void setRefParent(Node parent, Reference child) {
+    assert(child.parent == null);
+    child.parent = parent;
+  }
+
+  processFunctionDefinition(FunctionDefinition node) {
+    setParent(node, node.body);
+    node.parameters.forEach((Parameter p) => setParent(node, p));
+  }
+
+  // Expressions.
+
+  processLetPrim(LetPrim node) {
+    setParent(node, node.primitive);
+    setParent(node, node.body);
+  }
+
+  processLetCont(LetCont node) {
+    setParent(node, node.continuation);
+    setParent(node, node.body);
+  }
+
+  processInvokeStatic(InvokeStatic node) {
+    setRefParent(node, node.continuation);
+    node.arguments.forEach((Reference ref) => setRefParent(node, ref));
+  }
+
+  processInvokeContinuation(InvokeContinuation node) {
+    setRefParent(node, node.continuation);
+    node.arguments.forEach((Reference ref) => setRefParent(node, ref));
+  }
+
+  processInvokeMethod(InvokeMethod node) {
+    setRefParent(node, node.receiver);
+    setRefParent(node, node.continuation);
+    node.arguments.forEach((Reference ref) => setRefParent(node, ref));
+  }
+
+  processInvokeSuperMethod(InvokeSuperMethod node) {
+    setRefParent(node, node.continuation);
+    node.arguments.forEach((Reference ref) => setRefParent(node, ref));
+  }
+
+  processInvokeConstructor(InvokeConstructor node) {
+    setRefParent(node, node.continuation);
+    node.arguments.forEach((Reference ref) => setRefParent(node, ref));
+  }
+
+  processConcatenateStrings(ConcatenateStrings node) {
+    setRefParent(node, node.continuation);
+    node.arguments.forEach((Reference ref) => setRefParent(node, ref));
+  }
+
+  processBranch(Branch node) {
+    setParent(node, node.condition);
+    setRefParent(node, node.trueContinuation);
+    setRefParent(node, node.falseContinuation);
+  }
+
+  processTypeOperator(TypeOperator node) {
+    setRefParent(node, node.continuation);
+    setRefParent(node, node.receiver);
+  }
+
+  processSetClosureVariable(SetClosureVariable node) {
+    setParent(node, node.body);
+    setRefParent(node, node.value);
+  }
+
+  processDeclareFunction(DeclareFunction node) {
+    setParent(node, node.definition);
+    setParent(node, node.body);
+  }
+
+  // Definitions.
+
+  processLiteralList(LiteralList node) {
+    node.values.forEach((Reference ref) => setRefParent(node, ref));
+  }
+
+  processLiteralMap(LiteralMap node) {
+    node.values.forEach((Reference ref) => setRefParent(node, ref));
+    node.keys.forEach((Reference ref) => setRefParent(node, ref));
+  }
+
+  processCreateFunction(CreateFunction node) {
+    setParent(node, node.definition);
+  }
+
+  processContinuation(Continuation node) {
+    setParent(node, node.body);
+    node.parameters.forEach((Parameter param) => setParent(node, param));
+  }
+
+  // Conditions.
+
+  processIsTrue(IsTrue node) {
+    setRefParent(node, node.value);
+  }
+}
+
+class _ReductionKind {
+  final String name;
+  final int hashCode;
+
+  const _ReductionKind(this.name, this.hashCode);
+
+  static const _ReductionKind DEAD_VAL = const _ReductionKind('dead-val', 0);
+  static const _ReductionKind DEAD_CONT = const _ReductionKind('dead-cont', 1);
+  static const _ReductionKind BETA_CONT_LIN =
+      const _ReductionKind('beta-cont-lin', 2);
+  static const _ReductionKind ETA_CONT = const _ReductionKind('eta-cont', 3);
+
+  String toString() => name;
+}
+
+/// Represents a reduction task on the worklist. Implements both hashCode and
+/// operator== since instatiations are used as Set elements.

[sigurdm, 2014/08/01 09:09:41]

typo: instatiations => instantiations

[jgruber1, 2014/08/07 09:07:36]

Done.

+class _ReductionTask {
+  final _ReductionKind kind;
+  final Node node;
+
+  int get hashCode {
+    assert(kind.hashCode < (1 << 2));
+    return (node.hashCode << 2) | kind.hashCode;
+  }
+
+  _ReductionTask(this.kind, this.node);
+
+  bool operator==(_ReductionTask that) {
+    return (that.kind == this.kind && that.node == this.node);
+  }
+
+  String toString() => "$kind: $node";
+}
+
+/// A dummy class used solely to mark nodes as deleted once they are removed
+/// from a term.
+class _DeletedNode extends Node {
+  accept(_) => null;
+}